Radar apparatus

ABSTRACT

The present invention relates to a raster scanning radar display apparatus which is capable of improving the display of echo signals from objects in the vicinity of a ship on the display surface of an indicator. The raster scanning radar display apparatus includes a memory for storing echo signals based on one search signal, a sweep scanning device for drawing a plurality of imaginary sweeps which are parallel with each other in a display memory each time the pointing bearing of the antenna is varied and for generating signals representative of addresses of memory elements in a display memory scanned by the sweeps. The apparatus whites echo signals into the memory elements of the display memory scanned by one sweep and writes predetermined signals into the memory elements of the display memory scanned by other sweeps. The signals in the disply memory are read and displayed on an indicator.

TECHNICAL FIELD

The present invention relates to a radar apparatus which radiates searchsignals successively in different directions and receives echo signalscoming from the respective directions by means of a rotating antenna,writes the echo signals coming from a wide range of directions into adisplay memory to store the signals therein, and reads the signalstherefrom to supply them to an indicator so that surrounding conditionsaround the radar antenna are indicated on the display surface of theindicator. Particularly, the invention relates to a radar apparatuswhich writes echo signals caught by an antenna into a display memory inaccordance with a novel method, thereby improving the conveyance ofinformation relating to conditions around the vicinity of the own shipon the display surface of an indicator.

BACKGROUND ART

A conventional raster scanning radar is constructed as illustrated inFIG. 16. An antenna 901 rotates at a constant speed, radiates radarsearch signals successively in different directions and receives echosignals. A radar transmitting and receiving unit 907 receives echosignals; an A/D converter 902 converts the echo signals to digital data;and a primary memory 903 stores the resultant converted digital datatherein. The primary memory has a memory capacity to store all the echosignals based on one radar search signal corresponding to one radarsweep. A coordinates converter 904 converts the polar coordinates ofinformation in relation to surroundings about the radar antenna tocorresponding Cartesian coordinates based on bearing signals produced bythe radar transmitting and receiving unit 907 and generates signalsrepresentative of the addresses of corresponding memory elements of adisplay memory. The echo signals read from the primary memory arewritten into respective memory elements of a display memory 905 whichare disposed along an imaginary sweep drawn in correspondence to adirection in which echo signals come, the addresses of the memoryelements are identified by output signals of the coordinates converter904. FIG. 17 shows a relationship between imaginary sweeps traced in thedisplay memory 905 and some memory elements of the display memory whensignals read from of the primary memory are written into the memoryelements of the display memory 905. Every time the pointing bearing ofthe antenna 901 varies, relevant contents are refreshed in the displaymemory 905. Referring to FIG. 17, when a sweep SW1 is traced, echosignals based on one radar search signal and read from the primarymemory are successively written into memory elements M00, M01, M11, M12. . . disposed along the sweep. When a next sweep SW2 is drawn, echosignals supplied by the primary memory are written into memory elementsM00, M01, M11, M12, M22 . . . scanned by the sweep. When another sweepSW3 is traced, echo signals are written into memory elements M00, M01,M11, M12, M22 . . . disposed along the sweep. In the same way, whenanother sweep SW4 is run, echo signals are written into memory elementsM00, M10, M11, M21, M22 . . . disposed along the sweep. When anothersweep SW5 is drawn, echo signals in response to a following radar searchsignal and stored in the primary memory and read from the memory aresuccessively written into memory elements M00, M10, M11, M21, M22disposed along the sweep. When a further sweep SW6 is run, echo signalsread from the primary memory are successively written into memoryelements M00, M10, M11, M21 . . . disposed along the sweep. In the sameway, every time the pointing direction of the radar antenna is varied,an imaginary sweep is drawn in the display memory 905, echo signals readfrom the primary memory are written into the display memory. Anindicating device 906 comprises, for example, a raster-scannedcathode-ray tube, respective picture elements of which correspond torespective memory elements of the display memory 905 and indicates echosignals having come from a wide range of directions, read from thedisplay memory and applied at the input terminal thereof so thatsurrounding conditions are displayed on the display surface thereof.

With radar apparatuses, an object existing at a far distance isdisplayed as a relatively big image and the same object existing at ashort distance is displayed a small image and thus the same object at ashort distance is displayed as narrowly with respect to an angulardirection, since a radiation beam or a reception beam formed by a radarantenna has different widths depending on the distance from the antennaor is fan-shaped. Thus, echo signals generated at short distances fromthe antenna are harder to be recognized on an indicator. This situationalso becomes worse when a function for eliminating effects by searefections is used.

Further, with a raster scanning radar display apparatus, if the numberof memory elements are increased in order to improve quality ofdisplayed images, the dimensions of a memory element will become smalland thus images are displayed small in the vicinity of the center of thedisplay surface. This has presented difficulties in recognizing echosignals or actually causing failure in recognizing the echo signals.

An antenna having small outer dimensions for forming a radiation beamand a reception beam having a broad horizontal width has been used inorder to display as big images echo signals from objects in the vicinityof the own ship on the display surface of a radar indicator. With thisarrangement, angular resolution at long distances from the own shipdeteriorates on the indicating surface of an indicator and antennasensitivity will be lowered. Thus, maximum detection range of a radar ofthis type will be short.

With the conventional radar apparatus in the foregoing, echo signals arewritten a plurality of times during one rotation of the antenna intorespective memory elements of the display memory corresponding to areasin the vicinity of the radar antenna on the display surface of anindicator. Only the last echo signals written into the memory elementsare maintained therein and the other signals which had been previouslywritten therein are made invalid. If the last echo signals writtenlastly into memory elements are zero signals, there is presented such aproblem that no echo signals will be stored in the memory elements evenif echo signals have been previously written thereinto.

DISCLOSURE OF INVENTION

An object of the present invention is to provide a raster scanning radardisplay apparatus which is capable of improving the quality of theinformation conveyed by echo signals particularly, in the vicinity ofthe own ship being displayed on the display surface of a radarindicator.

Another object of the invention is to provide a radar apparatus whichexpands received echo signals in an angular direction and displays theresultant signals.

Another object of the invention is to provide a raster scanning radardisplay apparatus which draws a plurality of imaginary sweeps of adisplay memory when echo signals based on one radar search signal arewritten into the display memory, writes the echo signals into respectivememory elements disposed along one sweep and writes predeterminedsignals into respective memory elements disposed along the other sweeps.

Another object of the invention is to provide a raster scanning radardisplay apparatus which runs two imaginary sweeps of a display memorywhen echo signals based on one radar search signal are written into thedisplay memory, writes the echo signals into respective memory elementsarranged along one sweep and writes predetermined signals intorespective memory elements disposed along the other sweep.

Another object of the invention is to provide a raster scanning radardisplay apparatus which draws three imaginary sweeps of a display memorywhen echo signals based on one radar search signal are written into thedisplay memory, writes zero signals into memory elements thereofdisposed along the first sweep (i.e., the signals stored in therespective memory elements are erased) and writes predetermined signalsinto respective memory elements arranged along the second and thirdsweeps.

A further object of the invention is to provide a raster scanning radardisplay apparatus which runs two imaginary sweeps of a display memorywhen echo signals based on one radar search signal are written into thedisplay memory, writes zero signals into respective memory elementsthereof disposed along the first sweep (i.e., the signals stored in therespective memory elements are eliminated) and writes predeterminedsignals into respective memory elements disposed along the second sweep.

Still further object of the invention is to provide a raster scanningradar display apparatus which displays a sweep line marker (anindependent line marker irrespective of received echo signals) on thedisplay surface of an indicator.

A first feature of the present invention is to provide a raster scanningradar apparatus for radiating radar search signals successively indifferent directions by a rotating antenna, storing in a display memoryecho signals coming from respective directions, reading therefrom andsupplying the echo signals to an indicator to display conditionssurrounding the antenna on the display surface thereof. The presentinvention draws a plurality of imaginary sweeps which do not intersectwith each other in the display memory when echo signals are written intothe display memory, rotates these sweeps about the start points of thesweeps in a direction, writes into the memory elements scanned by onesweep echo signals or signals generated based on the echo signals andwrites predetermined signals into the memory elements scanned by theother sweeps.

A second feature of the present invention is to provide a radarapparatus for radiating radar search signals successively in differentdirections by a rotating antenna, storing in a display memory echosignals coming from respective directions, reading therefrom andsupplying the echo signals to an indicator to display conditionssurrounding the antenna on the display surface thereof. The presentinvention comprises a memory for storing echo signals based on onesearch signal, sweep moving means for drawing a plurality of imaginarysweeps which do not intersect with each other in the display memory andgenerating signals representative of addresses of the memory elementsscanned by these sweeps, writing means for writing echo signals intomemory elements of the display memory scanned by one sweep andpredetermined signals into the memory elements scanned by the othersweeps, reading means for reading signals from the display memory and anindicator for displaying the signals read from the display memory.

A third feature of the present invention is to provide a raster scanningradar display apparatus comprising a memory for storing echo signalsbased on one search signal, sweep moving means for drawing a pluralityof imaginary sweeps which do not intersect with each other in a displaymemory each time the pointing direction of the antenna is varied andgenerating signals representative of the addresses of the memoryelements scanned by the sweeps, judgement means for producingdistinguishing signals for distinguishing whether a present sweep forscanning memory elements of the display memory is a first one or asecond or a further one in the present scan of the radar antenna,writing means for, in response to output signals of the judgement means,writing echo signals supplied from the memory into memory elements ofthe display memory if one sweep for scanning memory elements of thedisplay memory is a first one and writing into memory elements of thedisplay memory scanned by one sweep signals having the larger values ofsignals stored in the memory and signals read from the memory elementsscanned by the sweep if the sweep for scanning the memory elements ofthe display memory is a second or a further one, writing means forwriting into the memory elements of the display memory scanned by theother sweep or sweeps than the one sweep signals having the largervalues of signals stored in the memory and signals read out of thememory elements scanned by the other sweep or sweeps, reading means forreading signals from the display memory and an indicator for displayingthe signals read out of the display memory.

A fourth feature of the present invention is to provide a rasterscanning radar display apparatus comprising a memory for storing echosignals based on one search signal, a display memory, erasing means forerasing all the signals stored in the display memory, address signalgenerating means for drawing one imaginary sweep or a plurality ofimaginary sweeps which ar in parallel with each other in the displaymemory each time the pointing bearing of the antenna is varied andgenerating signals representative of addresses of the memory elementsscanned by the sweeps, writing means for writing into the memoryelements of the display memory scanned by the one sweep o said pluralityof sweeps signals having the larger values of the signals stored in thememory and the signals read from the memory elements scanned by the onesweep or sweeps, reading means for reading signals from the displaymemory, and an indicator for displaying the signals read out of thedisplay memory.

A fifth feature of the present invention is to provide a raster scanningradar display apparatus comprising a memory for storing echo signalsbased on one search signal, first and second display memories, addresssignal generating means for drawing a plurality of imaginary sweepswhich are parallel with each other in the first and second displaymemories respectively each time the pointing bearing of the antenna isvaried and generating signals representative of the addresses of thememory elements scanned by the sweeps, zero signal generating means forgenerating zero signals, bearing signal generating means for generatingsignals representative of the pointing bearing of the radar antenna,writing means for, in response to output signals of the bearing signalgenerating means, writing alternately into the memory elements scannedby the plurality of sweeps in the first and second display memoriesrespectively signals having the larger values of the signals stored inthe memory and the signals read out of the memory elements scanned bythe plurality of sweeps and output signals of the zero signal generatingmeans, reading means for reading signals from the first and seconddisplay memories, and an indicator for displaying the signals read fromthe first and second display memories.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a block diagram of an embodiment according to the presentinvention;

FIG. 2 shows a block diagram of a portion of the embodiment shown inFIG. 1;

FIG. 3 shows a relationship between two independent sweeps scanningmemory elements in a display memory used in a embodiment according tothe present invention;

FIG. 4 shows a diagram for explaining a relationship between the startpoint of the first sweep and start points of the second sweep;

FIG. 5 shows a diagram for explaining a relationship between eightangular ranges obtained by dividing into eight the whole angular rangein which the pointing bearing of the radar antenna moves and startpoints of the second sweep;

FIG. 6 and FIG. 7 show diagrams for explaining relationships betweenfour angular ranges obtained by dividing into four the whole angularrange in which the pointing direction of the radar antenna moves andstart points of the second sweep;

FIG. 8 shows a block diagram of another embodiment according to thepresent invention;

FIG. 9 shows a detailed block diagram of an embodiment of a coordinatesconverting circuit and a start point signal generating circuit used in aradar apparatus according to the present invention;

FIG. 10 shows a diagram for explaining the relationship between thefirst and second sweeps and memory elements of the display memory (orpicture elements);

FIG. 11 shows a diagram for explaining operational timings of the firstand second sweeps drawn in an embodiment according to the presentinvention;

FIG. 12 shows a block diagram of an example of the start point signalgenerating circuit used in FIGS. 1, 8 or 15;

FIG. 13 shows a diagram for explaining a relationship between memoryelements of a display memory and operational sequence of writing signalsthereinto when echo signals are written into the display memory;

FIG. 14 shows a block diagram of another embodiment according to thepresent invention;

FIG. 15 shows a block diagram of another embodiment according to thepresent invention;

FIG. 16 shows a block diagram of a conventional raster scanning radarapparatus;

FIG. 17 shows a diagram for explaining the relationship betweenimaginary sweeps traced in the display memory and memory elementsthereof (or picture elements).

Throughout all the drawings, units having the same reference numeralsperform the same functions.

EMBODIMENTS

Referring to FIG. 10, there will be explained hereinafter a relationshipbetween two imaginary sweeps in parallel with each other drawn in adisplay memory used in an embodiment according to the present inventionand memory elements into which echo signals are written. "411, 412 . . .. . 421, 422 . . ." represent respective memory elements. "41" and "42"represent imaginary sweeps drawn in a display memory. The sweeps rotateabout their start points in a clockwise direction. Thus, the sweeps aredrawn successively in different directions from the start points. ,Whenthe second sweep 41 is run, echo signals read from a primary memory arewritten into memory elements 411, 412 . . . scanned by the sweep 41.When the first sweep 42 in parallel with the second sweep is drawn, echosignals read from the primary memory and signals read from the memoryelements 421, 422 . . . of the display memory are compared with oneanother, and signals having the larger values of the signals from theprimary memory and the signals from the display memory are written intothe memory elements 421, 422 . . . of the display memory scanned by thesweep 42. In this case, even if echo signals indicated in the vicinityof the own ship on the display surface of an indicator are written intocorresponding memory elements of the display memory a plurality of timesduring one rotation of the antenna, echo signals once written intomemory elements are not eradicated during the one rotation of theantenna.

It is also possible to write zero signals into the memory elements 411,422 . . . of the display memory scanned by the second sweep 41, when thesweep 41 is drawn. In this case, signals having previously been storedin the memory elements 411, 412 . . . are erased. Thus, a sweep linemarker (a line marker irrespective of received echo signals) can beindicated on the display surface of an indicator.

Referring to FIG. 1, a radar antenna 11 rotating at a speed radiatesradar search signals successively in different directions, receives echosignals based on the respective search signals and supplies the echosignals to an A/D converter 12. The A/D converter 12 converts inputanalog signals to digital signals and supplies the resultant convertedsignals to a sampling circuit 13. A primary memory 14 has a memorycapacity to store all the echo signals based on one rotation of a radarsearch signal and stores therein digital signals obtained with samplingsignals by the sampling circuit 13. A transmitting and receiving unit 60supplies signals 11a representative of the pointing direction of anantenna 11 to one input terminal of a coordinates converting circuit 17and to a start point signal generating circuit 19. The coordinatesconverting circuit 17 converts polar coordinates to Cartesiancoordinates and supplies a display memory 16 with signals representativeof the addresses of respective memory elements of the display memory 16.The coordinates converting circuit 17 supplies the display memory withsignals representative of addresses of the respective memory elementsscanned by the second sweep, when the second sweep is traced in thedisplay memory. The coordinates converting circuit 17 also supplies thedisplay memory with address signals for identifying memory elementsswept by the first sweep, when the first sweep is traced in the displaymemory. The start point signal generating circuit 19 supplies thecoordinates converting circuit 17 with signals for displacing the startpoint of the second sweep by one dot with respect to the start point ofthe first sweep. The coordinates converting circuit 17 applies signalsfor distinguishing sweeps at one input terminal of a comparison circuit15. The comparison circuit 15 writes signals of predetermined kinds intothe display memory based on sweep distinguishing signals fordistinguishing the first sweep or the second sweep supplied from thecoordinates converting circuits 17. When the second sweep is traced inthe display memory, signals supplied from the primary memory 14 arewritten into the memory elements scanned by the second sweep. When thefirst sweep is drawn, comparisons are made between signals read from thememory elements scanned by the first sweep and signals supplied from theprimary memory and signals having the larger values of signals from thememory elements and signals from the primary memory are written into thesame memory elements. An indicator 18 comprises a cathode-ray tube withall the picture elements thereof defined in Cartesian coordinates systemand corresponding to memory elements of the display memory respectivelyand indicates surroundings of the radar apparatus on its display surfacebased on signals read out of the display memory 16.

Next, operation of the embodiment according to the present inventionshown in FIG. 1 will be explained. Echo signals caught by the antenna 11resulting from each radar search signal are transmitted to the A/Dconverter 12 to be converted to digital signals. The resultant digitalsignals are sampled by the sampling circuit 13 and then stored in theprimary memory 14. While, the coordinates converting circuit 17 convertspolar coordinates to Cartesian coordinates based on signalsrepresentative of bearing of the antenna supplied from the transmittingand receiving unit 60 and supplies the display memory 16 with addresssignals for identifying memory elements in the display memory scanned bythe first sweep and the second sweep. In parallel with the generation ofthe address signals by the coordinates converting circuit 17, signalsstored in the primary memory 14 are supplied to the comparison circuit15 therefrom. Signals read from the memory elements of the displaymemory 16 scanned by the first sweep and the second sweep (i.e., memoryelements into which signals are newly written into) are supplied to thecomparison circuit 15. When the second sweep is drawn in the displaymemory, signals are supplied from the primary memory 14 through thecomparison circuit 15 to the display memory and are written into thememory elements scanned by the sweep and identified by address signalssupplied from the coordinates converting circuit 17. When the firstsweep is traced in the display memory, signal comparisons are made inthe comparison circuit 15 between signals supplied from the primarymemory 14 and signals read from memory elements into which signals willbe newly written, and signals having the larger values of these twokinds of signals are written into memory elements scanned by the sweepand identified by their addresses.

Next, there is explained interrelationships between two independentsweeps traced in the display memory used in the embodiment shown inFIG. 1. Referring to FIG. 3(b), the start point of the first sweep 21 isat the center, and the second sweep 22b is in parallel with the firstsweep. The start point "0" of the first sweep 21 and the start point221b of the second sweep 22b are spaced from each other by a distanceequivalent to the width of one memory element of the display memory. InCartesian coordinates, the start point 221b of the second sweep 22b withrespect to the start point "0" of the first sweep 21 can be at eightpositions 221 of picture elements in the neighborhood of the center "0"as shown in FIG. 4.

Referring to FIG. 5, while the pointing direction of the antenna is inan angular range designated as "M5-1", the position of a pictureelement, i.e., a memory element 221-1 will be the start point 221b ofthe second sweep 22b; while it is in an angular range designated as"M5-2", the position of a picture element 221-2 will be the start point221b of the second sweep 22b; and while it is in an angular rangedesignated as "M5-3", the position of a picture element 221-3 will bethe start point 221b of the second sweep 22b. The same relationshipholds true in the other angular ranges.

FIG. 2 shows a block diagram of an embodiment of the coordinatesconverting circuit 17 and the start point signal qenerating circuit 19used in the embodiment according to the present invention shown in FIG.1, with these circuits producing signals representative of the startpoint "0" of the first sweep 21 and the start point 221b of the secondsweep 22b.

Referring to FIG. 2, the coordinates converting circuit 17 is suppliedthrough signal lines 19a to 19d with signals for producing addresssignals representative of the start point "0" of the first sweep 21 andthe start point 221b of the second sweep 22b. The coordinates convertingcircuit 17 selects one of the memory elements 221-1, 221-2, 221-3, etc.,as the coordinates of the start point 221b which is spaced by one dotwith respect to the start point "0" of the first sweep 21 depending onlevels of the signals supplied through the four signal lines 19a through19d, when the second sweep 22b is run. One of the memory elements 221-1,221-2, 221-3, etc., is selected as the converted coordinates of thestart point 221b based on the signal 11a representative of the bearingof the antenna 11. The coordinates converting circuit 17 performscoordinates conversion without utilizing signals supplied from the startpoint signal generating circuit 19, i.e., not based on the signals sothat the start point will be the center "0", when the first sweep 21 isdrawn. When the second sweep 22b is traced, signals supplied through thefour signal lines 19a through 19d are utilized so that the start pointwill be at one of the points 221-1, 221-2, 221-3, etc., which are spacedby one dot with respect to the center "0".

When the first imaginary sweep is traced, prior to writing signals intomemory elements of the display memory 16, signals having been stored inmemory elements are read therefrom and compared with signals suppliedfrom the primary memory 14. The coordinates converting circuit 17 ismade to produce address signals to perform a writing operation in thedisplay memory. The display memory is supplied with instruction signalsfor performing a reading operation therein. Data signals read out of thedisplay memory 16 are stored in the comparison circuit 15 and comparedwith data signals supplied from the primary memory 14.

The sweep distinguishing signal supplied to the comparison circuit 15from the coordinates converting circuit 17 is at an "L" level when thesecond sweep is traced, and is at a "H" level when the first sweep isdrawn. When the sweep distinguishing signal is at a "H" level, datasignals read from the memory elements of the display memory 16 scannedby the first sweep (the data signals which have been previously storedin the memory elements into which signals will be newly written) arecompared with data signals supplied from the primary memory 14. Datasignals having the larger values of the signals from the display memoryand the signals from the primary memory are supplied to the displaymemory 16 and written into the memory elements. While, when the sweepdistinguishing signal is at an "L" level, the second sweep 22b is tracedand data signals read out of the primary memory 14 are written into thememory elements of the display memory 16 scanned by the sweep 22b.

In this way, a radar apparatus according to the present inventionradiates a radar search signal and receives echo signals by the antenna,stores the echo signals in the primary memory 14, reads the signals twotimes therefrom each time the pointing direction of the antenna isvaried, and writes two times the signals into the display memory 16.While, a conventional radar apparatus radiates a radar search signal andreceives echo signals by an antenna, stores the echos signals in theprimary memory, reads the signals on time therefrom and writes one timethe signals into the display memory each time the pointing direction ofthe antenna is changed.

When the second imaginary sweep is traced, zero signals can also bewritten into respective memory elements of the display memory scanned bythe sweep 41 so that a straight sweep line marker of narrow width can bedisplayed on the display surface of an indicator. This marker rotatesabout the start point thereof in synchronism with rotation of theantenna in an angular direction.

FIG. 9 shows a detailed block diagram of an X-coordinate generating unitin the coordinates converting circuit 17 and the start point signalgenerating circuit 19 (the electric circuit configuration of aY-coordinate generating unit is the same as that of the X-unit).

An addition and accumulation circuit 351 and an up-down counter 352 formthe coordinates converting circuit 17; and preset data generatingcircuit 353, a decoder 354 and counter 355 form the start point signalgenerating circuit 19.

The addition and accumulation circuit 351 is supplied with outputsignals 350a of a sine function generating circuit (not shown) at oneinput terminal thereof and with output signals of the addition andaccumulation circuit 351 at another input terminal thereof. An overflowsignal 351a representing that an accumulated value exceeds one issupplied to the up-down counter 352. The up-down counter 352 and theaddition and accumulation circuit 351 are supplied with clock signalsand reset signals. A counter 355 is supplied with angle clock signals350b and a heading signal 350c. Output signals of the counter 355 aresupplied to a decoder 354. Output signals of the decoder 354representative of the quadrant of a searched area are supplied to theup-down counter 352 and the preset data generating circuit 353. Outputsignals of the preset data generating circuit 353 are supplied to theup-down counter 352.

Operation of the circuit block diagram as shown in FIG. 9 will beexplained hereinafter.

The addition and accumulation circuit 351 adds the value of an outputsignal 350a of the sine function generating circuit to a previous valuein response to a clock signal corresponding to a detection range fromthe start point of the sweep, and transmits an overflow signal 351a tothe up-down counter 352 when a resultant added value exceeds one. At aninstant of start of the coordinates conversion, the value of theinternal adder of the addition and accumulation circuit is reset to zeroby a reset signal. The up-down counter 352 performs an up count or adown count each time an overflow signal 351a is produced and transmitsthe resultant count value representative of an X-coordinate to thedisplay memory 36 through the output line 352a. An up-count operation ora down-count operation is performed in accordance with a signalrepresentative of the quadrant of a search area supplied from thedecoder 354. At an instant of start of the coordinates conversion, theup-down counter 352 writes thereinto an output signal of the preset datagenerating circuit 353 and starts its counting operation with the valuewritten thereinto.

Output signals of the decoder 354 will represent a bearing range of thepointing direction of the antenna 11, since the output signals areobtained by decoding the output signals of the counter 355. The value ofpreset data generated by the preset data generating circuit 353 variesin accordance with output signals of the decoder 354 representative ofranges of pointing bearing of the antenna 11 as shown in FIG. 5.

As illustrated in FIG. 10, the second imaginary sweep 41 is firstly run,then the first imaginary sweep 42 is drawn from a start point which isspaced by one dot with respect to the start point of the first sweep. Inorder to always make the second sweep 41 be in parallel with the firstsweep 42, the start point 411 of the second sweep 41 is changeddepending on a quadrant represented by output signals of the decoder 354and to which the pointing bearing of the antenna 11 belongs. Asillustrated in FIG. 5, for example, if a quadrant to which the pointingbearing of the antenna 11 belong is a quadrant represented as "M5-1" inFIG. 5, it is controlled so that picture element 221-1 becomes the startpoint of the second sweep 41.

The writing operational timings in association with the sweeps 41 and 42is illustrated in FIG. 11. The writing operation is performed during atime period t1 of pulse "p" produced each time the pointing direction ofthe antenna 11 is varied. At a time instant "T1", in order to run thesecond sweep 41, the preset data generating circuit 353 transmits to theup-down counter 352 a value representative of a picture element 411corresponding to the start point of the second sweep 41. During a timeperiod "t2", coordinates conversion corresponding to the second sweep 41is performed, and data signals from the comparison circuit 15 arewritten into the display memory 16. At a time instant "T2", the presetdata generating circuit 353 transmits to the up-down counter 352 a valuecorresponding to the start point 421 of the first sweep 42. During atime period "t3", signals are written into respective memory elementsscanned by the first sweep 42 in the display memory 16.

FIG. 8 shows a block diagram of another embodiment according to thepresent invention.

Differences between the block diagram shown in FIG. 8 and the blockdiagram shown in FIG. 1 are that a memory access judgement signalqenerating circuit 20 is further provided and a comparison circuit 21performs operations different from those by the comparison circuit 15shown in FIG. 1.

Referring to FIG. 8, the radar antenna 11 radiates radar search signalssuccessively in different directions, receives echo signals based on therespective search signals and supplies the A/D converter 12 with theecho signals. The A/D converter 12 converts input analog signals todigital signals and supplies the sampling circuit 13 with the resultantconverted signals. The primary memory 14 has a memory capacity to storeecho signals corresponding to one sweep, with the echo signals producedbased on one radar search signal, and stores therein digital signalssampled by the sampling circuit 13. The transmitting and receiving unit60 supplies signals 11a representative of the pointing bearing of theantenna 11 to one input terminal of the coordinates converting circuit17 and to the start point signal generating circuit 19. The coordinatesconverting circuit 17 and the start point signal generating circuit 19are constructed as shown in FIG. 9. The coordinates converting circuit17 converts polar coordinates to Cartesian coordinates, supplies thedisplay memory 16 with signals representative of the addresses of memoryelements of the display memory 16 and transmits a carry pulse (anoverflow pulse or an underflow pulse) signal to the input terminal ofthe memory access judgement signal generating circuit 20. The memoryaccess judgement signal generating circuit 20 produces differentjudgement signals depending on the sweep number of times of, forexample, the second sweep 41 for scanning a memory element, for example,the memory element 412, with the second sweep 41 being of the two sweepsproduced in the coordinates converting circuit 17 as shown in FIG. 10.For example, it transmits a judgement signal "1" (New) to the comparisoncircuit 21 if the second sweep 41 for scanning the memory element 412 isa first one, and transmits another judgement signal "Old" thereto if thesecond sweep 41 is a second one or a further one. The construction ofthe memory access judgement signal generating circuit 20 is described inthe patent publication of an unexamined patent application No.62-212586. The memory access judgement signal generating circuit 20comprises a carry memorizing circuit 11, an up-down counter 12 and asame sweep address judgement circuit 14. The coordinates convertingcircuit 17 supplies the display memory with address signals foridentifying memory elements thereof scanned by the first sweep when thefirst sweep is traced in the display memory. The coordinates convertingcircuit 17 also supplies the display memory with signals representativeof addresses of the memory elements thereof scanned by the second sweepwhen the second sweep is traced in the display memory. The start pointsignal generating circuit 19 supplies the coordinates converting circuit17 with signals for displacing the start point of the second sweep byone dot with respect to the start point of the first sweep. Thecomparison circuit 21 writes signals supplied from the primary memoryinto the memory elements scanned by the second sweep when the judgementsignal "1" (New) is produced by the memory access judgement signalgenerating circuit 20, i.e., when the second sweep for scanning a memoryelement is a first one. While, the comparison circuit 21 comparessignals read from the memory elements scanned by the second sweep andsignals supplied from the primary memory and writes signals having thelarger values of the signals supplied from the memory elements and thesignals from the primary memory into the memory elements when the otherjudgement signal "Old" is produced by the memory access judgement signalgenerating circuit 20 i.e., when the second sweep 41 for scanning amemory element is a second one or a further one. When the first sweep 42is run, the comparison circuit 21 compares signals read from the memoryelements scanned by the first sweep with signals supplied from theprimary memory and writes into the same memory elements signals havingthe larger values of the signals supplied from the memory elements andthe signals supplied from the primary memory. The indicator 18 indicatesconditions surrounding the radar apparatus based on signals read out ofthe display memory 16. The timings for scanning the first and secondsweeps 42 and 41 are the same as those with the embodiment shown in FIG.1 as illustrated in FIG. 11.

Next, operation of the embodiment according to the present inventionshown in FIG. 8 will be explained hereinafter. Echo signals resultingfrom respective radar search signals and caught by the antenna 11 areconverted by the A/D converter 12 to digital signals, sampled by thesampling circuit 13 and stored in the primary memory 14. While, thecoordinates converting circuit 17 converts polar coordinates toCartesian coordinates based on bearing signals supplied from thetransmitting and receiving unit 60 and supplies the display memory 16with address signals for identifying the memory elements of the displaymemory scanned by the first and second sweeps. In parallel with theproduction of address signals by the coordinates converting circuit 17,signals having previously been stored in the primary memory 14 aretransmitted to the comparison circuit 21. There are supplied to thecomparison circuit 21 signals read from the memory elements (into whichsignals are newly written) of the display memory 16 scanned by the firstsweep and the second sweep. When the second sweep is traced in thedisplay memory 16 and the judgement signal "1" (New) is produced by thememory access judgement signal generating circuit 20, signals havingbeen stored in the primary memory are supplied to the display memoryfrom the comparison circuit 21 These echo signals are written intorespective memory elements of the display memory scanned by the secondsweep. When the second sweep is traced in the display memory 16 and theother judgement signal is produced by the memory access judgement signalgenerating circuit 20, signals having the larger values of signals readout of the memory elements scanned by the second sweep and signalssupplied from the primary memory. When the first sweep is traced afterthe second sweep, signals read from the memory elements of the displaymemory scanned by the first sweep are compared with signals suppliedfrom the primary memory and signals having the larger values of thesignals supplied from the display memory and the signals from theprimary memory are written into the same memory elements. Thus, forexample, if a memory element 412 shown in FIG. 10 is firstly scanned bythe second sweep 41, signals supplied from the primary memory arewritten into the memory element. There is written into a memory element422 scanned by the first sweep 42 and is contiguous to the memoryelement 412 a signal having the larger value of a signal read from thememory element 422 and a signal supplied from the primary memory so thatan echo signal is expanded in an angular direction and written into thedisplay memory. When the other judgement signal "Old" is produced fromthe memory access judgement signal generating circuit 20, signals readfrom the respective memory elements of the display memory scanned by thesecond sweep are compared by the comparison circuit 21 with signalssupplied from the primary memory and signals having the larger values ofthe signals from the display memory and the signals from the primarymemory are written into the same memory elements. Thus, if the secondsweep 41 scanning the memory element 412 shown in FIG. 10 is a secondone or a further one, signals having the larger values of the two kindsof signals from the two memories are written into the memory elements.There is written into a memory element 422 which is scanned by the firstsweep 42 and is contiguous to the memory element 412 signals having thelarger values of the signals read out of the memory element 422 and thesignals supplied from the primary memory so that echo signals areexpanded in an anqular direction and are written into the displaymemory. While the second sweep 41 and the first sweep 42 are rotated ina direction represented by an arrow "B" and scan other memory elementsin the display memory, the same operation is performed. If echo signalsare stored in the primary memory 14, the echo signals are expanded in anangular direction and are written into the display memory. Signals readfrom the display memory are indicated on the display surface of anindicator in a manner that echo signals caught by the antenna areexpanded in an angular direction.

It should be noted that although, with the embodiment shown in FIG. 1,the first sweep and the second sweep drawn in the display memory areparallel with each other, it should not be limited to only this one. Forexample, another relationship therebetween as shown in FIG. 3(a) is alsopossible, in which the start point of the first sweep 21 is at thecenter and the start point of the second sweep 22a is not at the centerand the first and second sweeps have different angles respectively withrespect to a reference line.

It should be noted that although the relationship between the startpoint of the first sweep and the start point of the second sweep in thedisplay memory is as shown in FIG. 5 in the foregoing embodiment, itshould not be limited to this one. It is also possible to divide thewhole angular range into four parts, as shown in FIG. 6. If the pointingbearing of the antenna is in an angular range designated as "M6-1", theposition of a picture element 221-1 is the start point of the secondsweep; and if the pointing bearing of the antenna is within a rangedesignated as "M6-3", the position of a picture element 221-3 is thestart point of the second sweep. Another relationship as shown in FIG. 7is also possible, in which if the pointing bearing of the antenna is inan angular range designated as "M7-2", the position of a picture element221-2 is the start point of the second sweep; and if the pointingdirection thereof is in a range designated as "M7-4", a picture element221-4 will be the start point of the second sweep. It should be notedthat the center "0" is the start point of the first sweep in FIG. 4,FIG. 5, FIG. 6 and FIG. 7.

It should be noted that although, with the embodiment shown in FIG. 8,two sweeps are drawn in the display memory 16, it should not be limitedto this one, and it is also possible to have three sweeps traced. Suchincrease of the number of the sweeps enables one to display echo signalsin the vicinity of the own ship on the display surface of an indicatorwith the echo signals greatly extended in an angular direction andfurther enlarged. FIG. 12 shows the construction of the preset datagenerating circuit 353 (FIG. 9) used in this embodiment. A selector 51is provided with signals 51a and 51b for displacing a sweep by one dotof picture element at the inputs thereof. The signals 51a and 51bdisplace the start point of the sweep in mutually opposite directionsrespectively. A selector 52 is provided with signals 52a and 52b fordisplacing a sweep by two dots of picture element at the inputs thereof.The signals 52a and 52b displace the start point of the sweep inmutually opposite directions. The selector 51 and the selector 52 aresupplied with signals 51c representative of quadrants of search bearing.The selector 51 and the selector 52 are controlled in response to thesignals 51c representative of quadrants of search bearing to select onesof the two signals applied thereto respectively and supply the selectedones to the input terminals of a selector 53. The selector 53 is alsosupplied with signals 53a which do not displace the start point of asweep. The selector 53 is also supplied with control signals 53b at itscontrol terminal. The selector 53 supplies in succession the up-downcounter 352 of the coordinates converting circuit 17 in response to thecontrol signals 53b with signals for not displacing the start point of asweep, signals for displacing it by one dot and signals for displacingit by two dots. In this case, for example, the first sweep is tracedstarting with the center "0", the second sweep is drawn in parallel withthe first sweep starting with a point which is spaced by one dot withrespect to the center "0", and the third sweep is run in parallel withthe first sweep starting with a point which is spaced by two dots withrespect to the center "0". When echo signals are written into thedisplay memory, firstly, signals supplied from the primary memory 14 aredirectly written into, or signals read out of the respective memoryelements scanned by the third sweep are compared with signals suppliedfrom the primary memory and signals having the larger values of thesignals from the memory elements and the signals from the primary memoryare written into the memory elements scanned by the third sweep,depending on a judgement signal produced by the memory access judgementsignal generating circuit 20. Then, comparisons are made between signalsread from the respective memory elements scanned by the second sweep andthe signals supplied from the primary memory and signals having thelarger values of the signals from the memory elements and the signalsfrom the primary memory are written into the memory elements scanned bythe second sweep. Then, comparisons are made between signals read fromthe respective memory elements of the display memory scanned by thefirst sweep and the signals supplied from the primary memory, andsignals having the larger values of the signals from the display memoryand the signals from the primary memory are written into the memoryelements scanned by the first sweep. It is to be noted that with theembodiment according to the present invention shown in FIG. 8 and FIG.9, the imaginary first and second sweeps are always drawn in the displaymemory and received echo signals are displayed as expanded in a angulardirection. As another embodiment, a mode setting unit is provided andthere is selected by the mode setting unit one of "Mode-1" and "Mode-2",with the Mode-1 for scanning memory elements by two imaginary sweeps ofthe display memory and storing echo signals therein as expanded in anangular direction and with the Mode-2 for scanning memory elements byone sweep of the display memory and storing echo signals therein withoutstretching the signals in an angular direction. When the Mode-1 isselected, the operation is the same as that of the embodiment shown inFIG. 8, as explained in the foregoing. When the Mode-2 is selected, eachtime the pointing direction of the antenna is varied, signals aresupplied to the up-down counter 352 from the preset data generatingcircuit 353, shown in FIG. 9, for always placing the start point of thesweep at the center "0", thereby setting the value of the up-downcounter to be zero. As a result, received echo signals are displayedwithout being expanded in an angular direction.

It should be noted that with the embodiment shown in FIG. 8, the lengthof the first sweep always corresponds to the search range selected, andall the incoming signals are extended in an angular direction and theecho signals are displayed as enlarged. As another embodiment, thelength of the sweep is made to correspond to a short distance range fromthe own ship o the display surface of an indicator and thus only theecho signals indicated in the vicinity of the own ship on the displaysurface of the indicator are extended in an angular direction. Echosignals to be indicated at portions of the display surface at longdistances away from the own ship are written into the memory elementsscanned by one sweep in the display memory and are displayed on anindicator without being expanded in an angular direction.

It should be noted that with the embodiment shown in FIG. 8, the memoryaccess judgement signal generating circuit 20 is provided, and twoimaginary sweeps are drawn in the display memory 16 so that receivedecho signals are extended in an angular direction and displayed asexpanded. It is also possible to extend received echo signals in anangular direction and display the signals as expanded without having thememory access judgement signal generating circuit This can be achievedby a radar apparatus having the construction thereof illustrated as theblock diagram shown in FIG. 1. But, the coordinates converting circuit17, the start point signal generating circuit 19 and the comparisoncircuit 15 operate in a different manner than explained in theforegoing. The coordinates converting circuit 17 scans memory elementswith the first, second and third sweeps which are in parallel with eachother and writes signals into memory elements scanned by the sweeps. Thestart point signal generating circuit 19 supplies the coordinatesconverting circuit 17 with signals for determining the start points ofthe first, second and third sweeps. The start point of the first sweepis at the center "0" , the start points of the second and third sweepsare points spaced by one dot and two dots of picture element of theindicator respectively with respect to the center "0". The coordinatesconverting circuit 17 also transmits to the comparison circuit 15distinguishing signals for distinguishing the first, second and thirdsweeps from one another. When the third sweep is run, zero signals fromthe comparison circuit 15 are written into memory elements of thedisplay memory scanned by the sweep. When the second sweep is drawn,there are written into memory elements of the display memory scanned bythe sweep signals having the larger values of signals read from thememory elements and signals supplied from the primary memory. Then, whenthe first sweep is traced, there are written into the memory elements ofthe display memory scanned by the sweep, signals having the largervalues of the signals read from the memory elements and the signalssupplied from the primary memory.

It should be noted that although in the foregoing embodiments, as shownin FIG. 10, the second sweep 41 is firstly drawn in the display memoryto successively identify memory elements, and then the first sweep istraced to identify memory elements which are spaced by one dot ofpicture element with respect to the memory elements identified by thesecond sweep. The method for identifying memory elements of the displaymemory is not limited to this one. The same can be achieved by anothermethod illustrated in FIG. 13. For example, memory elements can beidentified in the order of M00, M10, M01, M11, M02, M12, M03, M13 . . .. . . , which results in the same as the memory elements identified bythe two sweeps explained in the foregoing.

FIG. 14 shows another embodiment according to the present invention.

This embodiment of the present invention scans each of memory elementscorresponding to areas in the vicinity of the antenna in the displaymemory with a plurality of sweeps during one rotation of the antenna andmakes it possible to maintain storing in a memory element echo signalshaving been previously written even if zero signals are supplied to thedisplay memory when the last sweep scans the memory element. One of thedifferences between the embodiment shown in FIG. 1 and this embodimentis that two display memories are provided and the start point signalgenerating circuit 19 becomes unnecessary.

The coordinates converting circuit 17 supplies the display memory 16 andthe display memory 65 with the same address signals to identify memoryelements in these display memories with the memory elements in onedisplay memory corresponding to the memory elements in the other displaymemory. The display memory 16 and the display memory 65 draw respectivesweeps therein each time the pointing direction of the antenna is variedto scan memory elements in the two display memories respectively withthe memory elements in one display memory corresponding to the memoryelements in the other display memory so that signals supplied from aselector 66 are written into the memory elements of the display memoriesrespectively. The display memory 16 and the display memory 65 transmitsignals stored therein to the input terminal of a comparison circuit 67through a selector 70. The selector 70 supplies the comparison circuit67 with signals alternately from the display memory 16 and the displaymemory 65 in response to bearing signals produced by the transmittingand receiving unit 60, for example, heading line signals. The comparisoncircuit 67 compares echo signals supplied from the primary memory 14with signals read from the memory elements of the display memory 16 orthe display memory 65 and supplies signals having the larger values ofthe signals from the primary memory and the signals from the displaymemories to one input terminal of the selector 66. A zero signalgenerating circuit 68 generates zero signals and supplies the signals tothe other input terminal of the selector 66. The selector 66 suppliesoutput signals of the comparison circuit 67 or zero signals suppliedfrom the zero signal generating circuit 68 alternately in response tobearing signals from the transmitting and receiving unit 60 to one inputterminals of the display memory 16 and the display memory 65. Forexample, during one rotation of the antenna 11, the selector 66 writesoutput signals of the comparison circuit 67 into the display memory 16and zero signals supplied from the zero signal generating circuit 68into the display memory 65. In this case, signals produced by thedisplay memory 16 are supplied to the comparison circuit 67 through theselector 70. During one rotation of the antenna after a heading linesignal is supplied to the selectors 66 and 70 from the radartransmitting and receiving unit 60, output signals of the comparisoncircuit 67 are written into the display memory 65, and zero signalsproduced by the zero signal generating circuit 68 are written into thedisplay memory 16. In this case, signals produced by the display memory65 are supplied to the comparison circuit 67 through the selector 70.Signals simultaneously read out of the display memory 16 and the displaymemory 65 are supplied to the indicator 18 through an OR circuit 69.

FIG. 15 shows another embodiment according to the present invention.

This embodiment of the present invention enables one to indicate echosignals received by the radar antenna as expanded in an angulardirection, the same as the embodiment shown in FIG. 8. The differencebetween the embodiment shown in FIG. 8 and the one in FIG. 15 is thattwo display memories are provided and the memory access judgement signalgenerating circuit 20 becomes unnecessary. The coordinates convertingcircuit 17 supplies the display memory 16 and the display memory 75 withthe same address signals to identify memory elements with the memoryelements in one display memory corresponding to the memory elements inthe other display memory. The display memory 16 and the display memory75 draw two sweeps respectively each time the pointing bearing of theantenna is varied to scan memory elements corresponding to each other sothat signals supplied from the selector 76 are written into the memoryelements of the display memories. The display memory 16 and the displaymemory 75 transmit signals stored therein to the input terminal of thecomparison circuit 77 through the selector 80. The selector 80 suppliesthe comparison circuit 77 with signals from the display memory 16 or thedisplay memory 75 alternately in response to bearing signals produced bythe radar transmitting and receiving unit 60, for example, heading linesignals. The comparison circuit 77 compares signals supplied from theprimary memory 14 with signals read from the memory elements of thedisplay memory 16 or the display memory 75 scanned by the imaginarysweeps and supplies signals having the larger values of the signals fromthe primary memory and the signals from the display memories to oneinput terminal of the selector 76. A zero signal generating circuit 78produces zero signals and supplies the signals to the other inputterminal of the selector 76. The selector 76 supplies output signals ofthe comparison circuit 77 or zero signals produced by the zero signalgenerating circuit 78 to signal input terminals of the display memory 16or the display memory 75 alternately in response to bearing signals, forexample, heading line signals. For example, during one rotation of theantenna 11, the selector 76 writes output signals of the comparisoncircuit 77 into the display memory 16 and zero signals produced by thezero signal generating circuit 78 into the display memory 75. In thiscase, signals from the display memory 16 are transmitted to thecomparison circuit 77 through the selector 80. Next, during one rotationof the antenna after a heading line signal is supplied to the selectors76 and 80 from the radar transmitting and receiving unit 60, outputsignals of the comparison circuit 77 are written into the display memory75, and zero signals produced by the zero signal generating circuit 78are written into the display memory 16. In this case, signals producedby the display memory 75 are transmitted to the comparison circuit 77through the selector 80. Signals simultaneously read from the displaymemory 16 and the display memory 75 are supplied to the indicator 18through an OR circuit 79.

It should be noted that although the start point of the first sweep ofthe first, second and third sweeps drawn in the display memory is at thecenter in the foregoing embodiments, the start point of the second sweepcan be placed at the center "0". It can be appropriately determined thatthe start point of one of the first, second and third sweeps is placedat the center "0", and another sweep or the other sweeps are positionedat the right side or the left side of the sweep.

It should be noted that the embodiments in the foregoing are only someembodiments in accordance with the present invention. It is apparentthat a man skilled in the art can easily create other embodiments ormodifications without departing the scope and the spirit of the presentinvention.

What is claimed is:
 1. A raster scanning radar display apparatus forradiating radar search signals successively in different directions,storing in a display memory echo signals coming from respectivedirections, reading therefrom, and supplying signals to be displayed asconditions surrounding an antenna, comprising:a memory for storing echosignals based on one search signal; write address signal generatingmeans for drawing a plurality of imaginary sweeps which do not intersectwith each other in the display memory each time a pointing bearing ordirection of the antenna is varied and for generating signalsrepresentative of addresses of memory elements in the display memoryscanned by the sweeps; writing means for writing echo signals into thememory elements of the display memory scanned by one sweep and forwriting predetermined signals into memory elements scanned by an othersweep or other sweeps; reading means for reading signals from thedisplay memory; and an indicator for displaying the signals read fromthe display memory as conditions surrounding the antenna.
 2. The rasterscanning radar display apparatus as claimed in claim 1 wherein twosweeps are drawn.
 3. The raster scanning radar display apparatus asclaimed in claim 2 wherein the two sweeps are in parallel with eachother.
 4. The raster scanning radar display apparatus as claimed inclaim 3 wherein echo signals are written into memory elements scanned byone sweep, and predetermined signals are written into memory elementsscanned by another sweep.
 5. The raster scanning radar display apparatusas claimed in claim 4 wherein zero signals are written into the memoryelements scanned by the one sweep.
 6. The raster scanning radar displayapparatus as claimed in claim 4 wherein signals having larger values asbetween signals stored in the memory and signals read from the memoryelements of the display memory scanned by the other sweep are written inthe display memory.
 7. The raster scanning radar display apparatus asclaimed in claim 1 wherein three straight sweeps are drawn in thedisplay memory which are parallel with each other.
 8. The rasterscanning radar display apparatus as claimed in claim 7 wherein zerosignals are written into memory elements of the display memory scannedby the third sweep, and larger values of signals read from the memoryelements of the display memory and echo signals supplied from the memoryare written into memory elements of the display memory scanned by thesecond and first sweeps.
 9. The raster scanning radar display apparatusas claimed in claim 1 wherein start points of respective sweeps arespaced from one another by one picture element.
 10. A raster scanningradar display apparatus for radiating radar search signals successivelyin different directions by a rotating antenna, storing in a displaymemory echo signals coming from respective directions, readingtherefrom, and supplying signals to be displayed as conditionssurrounding the rotating antenna, comprising:a memory for storing echosignals based on one search signal; sweep scanning means for drawing aplurality of imaginary sweeps which do not intersect with each other inthe display memory each time a pointing bearing of the rotating antennais varied and for generating signals representative of addresses ofmemory elements in the display memory scanned by the sweeps; judgementmeans for determining whether a present sweep for scanning memoryelements of the display memory is a first sweep or a second sweep or afurther sweep; first writing means, in response to said judgement means,for writing echo signals supplied from said memory into memory elementsof the display memory if a sweep for scanning the memory elements is afirst sweep and for writing into memory elements of the display memory,scanned by a sweep, signals having larger values as between signalsstored in said memory and the signals read from the memory elements ofthe display memory scanned by the sweep if the sweep is a second sweepor a further sweep; second writing means for writing into the memoryelements of the display memory scanned by a sweep other than said firstsweep signals having larger values as between the signals stored in saidmemory and the signals read from the memory elements of the displaymemory scanned by the other sweeps; reading means for reading signalsfrom the display memory; and an indicator for displaying the signalsread from the display memory.
 11. The raster scanning radar displayapparatus as claimed in claim 10 wherein two imaginary straight sweepswhich are parallel with each other are drawn in the display memory. 12.The raster scanning radar display apparatus as claimed in claim 10wherein three straight sweeps which are in parallel with each other aredrawn in the display memory.
 13. The raster scanning radar displayapparatus as claimed in claim 10 wherein start points of the respectivesweeps are spaced from one another by one picture elements.
 14. A rasterscanning radar display apparatus for rotating an antenna, radiatingradar search signals successively in different directions by theantenna, storing in a display memory echo signals coming from respectivedirections, reading therefrom, and supplying signals to be displayed asconditions surrounding the antenna, comprising:a memory for storing echosignals based on one search signal; means for writing into memoryelements of the display memory signals stored in said memory and signalsgenerated based on the signals stored in said memory each time apointing bearing or direction of the antenna is varied; reading meansfor reading signals from the display memory; and an indicator forindicating the signals read from the display memory.
 15. A rasterscanning radar display apparatus for rotating an antenna, radiatingradar search signals successively in different directions by theantenna, storing in a display memory echo signals coming from respectivedirections, reading therefrom, and supplying signals to be displayed asconditions surrounding the antenna, comprising:a memory for storing echosignals based on one search signal; erasing means for erasing allsignals stored in the display memory; address signal generating meansfor drawing one imaginary sweep or a plurality of imaginary sweeps whichare in parallel with each other in the display memory each time apointing bearing or direction of the antenna is varied and forgenerating signals representative of addresses of memory elements in thedisplay memory scanned by the sweep or sweeps; writing means for writinginto the memory elements of the display memory scanned by said oneimaginary sweep or said plurality of imaginary sweeps signals havinglarger values as between signals stored in said memory and signals readfrom the memory elements in the display memory scanned by the sweep orsweeps; reading means for reading signals from the display memory; andan indicator for displaying the signals read from the display memory.16. The raster scanning radar display apparatus as claimed in claim 15wherein said plurality of imaginary sweeps are two sweeps.
 17. A rasterscanning radar display apparatus for rotating an antenna, radiatingradar search signals successively in different directions by theantenna, storing echo signals coming from respective directions, readingtherefrom, comprising:a memory for storing echo signals based on onesearch signal; a first and second display memory; address signalgenerating means for drawing a plurality of imaginary sweeps which areparallel with each other in said first and second display memory eachtime a pointing bearing or direction of the antenna is varied and forgenerating signals representative of addresses of memory elements insaid first and second display memory scanned by the sweeps; zero signalgenerating means for generating zero signals; bearings signal generatingmeans for generating signals representative of the pointing bearing ofthe radar antenna; writing means, in response to said bearing signalgenerating means, for writing alternately into the memory elementsscanned by said plurality of sweeps in said first and second displaymemory, respectively, signals having larger values as between signalsstored in said memory and signals read from the memory elements scannedby said plurality of sweeps and output signals of said zero signalgenerating means; reading means for reading signals from said first andsecond memory; and an indicator for displaying the signals read fromsaid first and second display memory.
 18. A raster scanning radardisplay apparatus for rotating an antenna, radiating radar searchsignals successively in different directions by the antenna, storingecho signals coming from respective directions, reading therefrom, andsupplying signals to be displayed as conditions surrounding the antenna,comprising:a memory for storing echo signals based on one search signal;a first and second display memory; address signal generating means fordrawing one imaginary sweep in said first and second display memory,respectively, each time a pointing bearing or direction of the antennais varied and for generating signals representative of addresses ofmemory elements of said first and second display memory scanned by thesweeps; zero signal generating means for generating zero signals;bearing signal generating means for generating signals representative ofthe pointing bearing or direction of the radar antenna; writing means,in response to said bearing signal generating means, for writingalternately into the memory elements scanned by said sweeps in saidfirst and second display memory, respectively, signals having largervalues as between signals stored in said memory and signals read fromthe memory elements scanned by said sweeps and output signals of saidzero signal generating means; reading means for reading signals fromsaid first and second memory; and an indicator for displaying thesignals read from said first and second display memory.
 19. A method fordisplaying echo signals of radar search signals radiated successively indifferent directions on a raster scanned displayed surface, comprisingthe steps of:(a) drawing a plurality of sweeps which do not intersectwith each other when echo signals are written into a display memory; (b)rotating these sweeps about start points of the sweeps in a direction;(c) writing into memory elements of the display memory scanned by onesweep echo signals or signals generated based on the echo signals; and(d) writing predetermined signals into the memory elements of thedisplay memory scanned by other sweeps.
 20. A raster scanning radardisplay apparatus for rotating an antenna, radiating radar searchsignals successively in different directions by the antenna, storing ina display memory echo signals coming from respective directions, readingtherefrom, and supplying signals to be displayed as conditionssurrounding the antenna, comprising:a memory for storing echo signalsbased on one search signal; first sweep scanning means for drawing onesweep in the display memory each time a pointing bearing of the antennais varied and for generating signals representative of addresses ofmemory elements of the display memory scanned by the sweep; second sweepscanning means for drawing a plurality of sweeps which are in parallelwith each other in the display memory each time the pointing bearing ofthe antenna is varied and for generating signals representative ofmemory elements of the display memory scanned by the sweeps; selectingmeans for selecting either said first sweep scanning means or saidsecond sweep scanning means; judgement means for determining whether asweep by said first sweep scanning means or a sweep by said second sweepscanning means is a first sweep or a second sweep or a further sweep;first writing means, in response to said judgement means, for writingecho signals supplied from said memory, if the sweep by said first sweepscanning means or the sweep by said second sweep scanning means is afirst sweep, into the memory elements of the display memory scanned bythe sweep, and if the sweep by said first sweep scanning means or thesweep by said second sweep scanning means is a second sweep or a furthersweep, writing into the memory elements scanned by the sweep signalshaving larger values as between signals stored in said memory andsignals read from the memory elements scanned by the sweep; secondwriting means for writing into the memory elements of the display memoryscanned by other than said first sweep by said second sweep scanningmeans signals having larger values as between signals stored in saidmemory and signals read from the memory elements scanned by the othersweeps; reading means for reading signals from the display memory; andan indicator for displaying the signals read from the display memory.